A comparative study on characteristics of composite (Cr3C2-NiCr) clad developed through diode laser and microwave energy

A typical ferrite/martensitic heat-resistant steel (T91) is widely used in reheaters, superheaters and power stations. Cr3C2-NiCr-based composite coatings are known for wear-resistant coatings at elevated temperature applications. The current work compares the microstructural studies of 75 wt% Cr3C2- 25 wt% NiCr-based composite clads developed through laser and microwave energy on a T91 steel substrate. The developed clads of both processes were characterized through a field emission scanning electron microscope (FE-SEM) attached with energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and assessment of Vickers microhardness. The Cr3C2-NiCr based clads of both processes revealed better metallurgical bonding with the chosen substrate. The microstructure of the developed laser clad shows a distinctive dense solidified structure, with a rich Ni phase occupying interdendritic spaces. In the case of microwave clad, the hard chromium carbide particles consistently dispersed within the soft nickel matrix. EDS study evidenced that the cell boundaries are lined with chromium where Fe and Ni were found inside the cells. The X-ray phase analysis of both the processes evidenced the common presence of phases like chromium carbides (Cr7C3, Cr3C2, Cr23C6), Iron Nickel (FeNi3) and chromium-nickel (Cr3Ni2, CrNi), despite these phases iron carbides (Fe7C3) are observed in the developed microwave clads. The homogeneous distributions of such carbides in the developed clad structure of both processes indicated higher hardness. The typical microhardness of the laser-clad (1142 ± 65HV) was about 22% higher than the microwave clad (940 ± 42 HV). Using a ball-on-plate test, the study analyzed microwave and laser-clad samples' wear behavior. Laser-cladding samples showed superior wear resistance due to hard carbide elements. At the same time, microwave-clad samples experienced more surface damage and material loss due to micro-cutting, loosening, and fatigue-induced fracture.


Experimental details
The material details, experimental procedure and characterization techniques used in the current work have been briefly discussed in the following sections.
Substrate and powder materials. In the current work, commercially available 75 wt% chromium carbide-25 wt% Nickel Chromium (75Cr 3 C 2 -25NiCr) composite powder (Make: Oerlikon metco (Woka 7202)) having a particle size of 45 µm was used to develop clads on T91 ferritic alloy steel. A microstructure of substrate T91 is shown in Fig. 1a. The particles of the clad powder had a spherical form. Figure 1b shows the typical shape of the unprocessed clad powder used for deposition. XRD pattern of Cr 3 C 2 -NiCr composite powder is illustrated in Fig. 1d. This shows the dominant presence of Cr 3 C 2 along with NiCr. The Cr 3 C 2 particles are liable for imparting higher hardness, while NiCr acts as a binder that offers greater matrix strength through its excellent adhesion properties and carbide wetting. The substrates were machined to the desired dimensions from the T91 steel plate. The chemical composition of chosen clad powder (Cr 3 C 2 -NiCr) and T91 substrate is presented in Table 1.
The XRD spectrum of the substrate T91 shows the major dominance of ferrite iron Fig. 1c.
The clad powder and substrate preparation are also important in the development process. Therefore, the substrate was polished with a standard metallurgical process and cleaned with acetone before powder deposition. The clad powder was warmed at 200 °C for 24 h in a normal muffle furnace to eliminate possible moisture content. Cr 3 C 2 -NiCr-based composite clads are developed through two different processes described as follows.
Laser cladding process. Cr 3 C 2 -NiCr-based composite clads were developed through laser energy on the T91 substrate. The laser cladding experimental setup equipped with a 10 kW diode laser consists of fiber delivery  www.nature.com/scientificreports/ and an optic head system placed on a 6-axis robot with a square spot size of 6 mm. An off-axis powder-feeding nozzle assembly was employed to feed the powder on the substrate during laser interaction with argon gas. A vertical distance between the substrate and a laser beam was maintained by 14 mm. To optimize the process parameters, numerous experimental tests were carried out. Finally, laser clads were developed with 2000 W power by maintaining a 5 mm/s scanning speed with a powder feed rate of 8 g/min was retained. A Typical laser clad experimental system used is shown in Fig. 2.
Microwave cladding process. In this process, Cr 3 C 2 -NiCr based clads developed through microwave energy on substrate T91. Before deposition of the clad powder, the flat T91 substrate was thoroughly cleaned using alcohol, ensuring its cleanliness. Initially, the clad powder was mixed with Araldite binder to prepare a slurry; the prepared slurry of clad powder was applied uniformly on a substrate with an approximate thickness of 1 mm. Experimental trials were carried out using a conventional microwave oven, with a 99% pure alumina plate (insulation), approximately 0.5 mm thick, kept on the slurry of clad powder applied on the substrate. The alumina plate performs as a separator between clad powder and the susceptor. The charcoal powder was used as a susceptor which initiates heating and helps to increase the temperature of the clad powder particles beyond its critical level. Once the clad powder reaches its critical temperature, these particles couple with an incident of microwave radiation, further leading to heated up and melting. The metallic substrate was placed on the refractory base. A highly microwave-absorbent material called a susceptor was employed to raise the temperature of the powder particles. Microwave hybrid heating was then used to melt the preplaced powder. Once the experimental configuration was ready, the arrangement of the hybrid heating setup was placed on the turn table and exposed to microwave radiation at the domestic microwave oven. The schematic of the experimental setup is shown in Fig. 3 Characterization of clads. The Cr 3 C 2 -NiCr based clads developed through the above processes were sectioned across the thickness and were hot mounted in epoxy. The mounted samples were then polished using standard metallographic techniques. XRD phase analysis was carried out through a Rigaku diffractometer using Cu Kα X-ray at room temperature. The scan rate was 1°/min while the 10°-120° scan range was maintained. Microstructure and EDS studies were carried out through a field emission scanning electron microscope. Microhardness of the clads was carried out at 500 g load with 15 s dwell time through a Vickers' microhardness tester (VMHT Micro Hardness Tester). A consistent distance of 100 µm was used for all microhardness indentations.
Wear test. The wear test was conducted using linear reciprocatory ball-on-plate tribometers (THT1000 and TRB3, Anton Paar, Austria, ASTM G133) to evaluate the wear behavior of rough polished wire-cut microwave-clad and laser-clad samples. The dimensions of the samples were 10 mm × 10 mm × 6 mm (length × width × height). Table 2 provides the details of the wear parameters employed during the test. To examine the fretting/fatigue The worn clads were subjected to SEM analysis to investigate and identify the associated wear mechanisms after the wear test.

Mechanism of clad development
The modified surface behavior of engineering components across these two significant cladding methods can be understood well by attributing to their formation structure. The principles of surface development are illustrated schematically by a "single-particle processing" concept in Fig. 4. A laser cladding and sprayed powder produces a high-quality clad layer with minimal dilution. The powder particles are transported into the melt pool through a carrier gas and focussed at an angle of 38°-45° towards the target substrate. Complete melting and solidifying result in the dense microstructure. However, in the laser cladding process, the energy must be high enough to melt the powder particles and low to avoid the substrates' melting. The powder particles striking the substrate outside the melt pool bounce, but the particles striking the melt pool lead to melting completely. In laser clads, there are some concerns about residual stress development due to rapid solidification cracking, high thermal gradient and porosity 27 . Therefore, microcracks and porosity (Fig. 5a) cause spalling to the laser clads under severe working circumstances. In the microwave cladding process, heat is produced within powder particles due to dielectric losses, which further cause the volumetric nature of heating and subsequent melting. The molten clad powder particles further cause to raise the substrate temperature to its melting point and get fused (Fig. 4.). Upon solidification, a better-developed clad structure with uniform and dense microstructure, free from solidification cracking with negligible porosity can be seen (Fig. 5b).

Results and discussion
Cr 3 C 2 -NiCr-based composite clads are developed through laser and microwave energy and are characterized through various techniques, and the findings are discussed in the subsequent sections.
XRD phase analysis. An XRD spectrum of clads developed through laser energy and microwave energy is shown in Fig. 6. The clad spectrum of both the process evidenced the common presence of phases like chromium carbides (Cr 7 C 3 Cr 3 C 2 , Cr 23 C 6 ), Iron Nickel (FeNi 3 ) and chromium-nickel (Cr 3 Ni 2 , CrNi). However, these iron carbides (Fe 7 C 3 ) phases are observed in clads developed through microwave energy. XRD spectrum of laser clad surface (Fig. 6a) reveals that most peaks are chromium carbides, and minor peaks like iron-nickel and chromium-nickel are observed. It is clear that the decarburization of Cr 3 C 2 results in the formation of chromium carbides such as Cr 7 C 3 and Cr 23 C 6 . Cr 7 C 3 is primarily formed from the decarburization of Cr 3 C 2 due to the mas-  www.nature.com/scientificreports/ sive melting state of heating at the laser cladding process. This is confirmed by the fact that many Cr 7 C 3 are found around the Cr 3 C 2 particles 28 . Thus, a proportion of carbon is ideally precipitated as Cr 23 C 6 . The high-temperature melting of the laser cladding process leads to cause the partial dissolution of the primary Cr 3 C 2 , and this might be one of the major possibilities for the formation of types of chromium carbides. Such behavior enhances the carbon and chromium content of the melt pool, which stimulates the formation of many other carbide phases during most of the non-equilibrium cooling process. The formation of various chromium carbides (Cr 3 C 2 and Cr 23 C 6 ) has also been recorded earlier for the laser cladding of Ni60-Cr 3 C 2 10 . Minor peaks such as chromiumnickel (Cr 3 Ni 2 , CrNi) are formed by the NiCr binder.
This NiCr binder will likely melt initially and crystallizes some chromium carbide in a liquid phase (Cr 3 Ni 2 ) that may be rich in Cr, and C. Iron-nickel (FeNi 3 ) may be due to diffusion of iron elements from the target surface  www.nature.com/scientificreports/ to the clad, which is a clear proof of metallurgical bonding of substrate to clad. Another interesting observation seems to be the development of ferromagnetic FeNi 3 intermetallic, even though chosen clad powder was ironfree (Table 1). These results indicate the dilution of elements in which iron has been diluted from the substrate. The formation of this type of intermetallic was also reported earlier 12 . The typical XRD spectra of composite clad (Cr 3 C 2 -NiCr) developed through microwave energy are shown in Fig. 6b. The existence of different phases such as chromium carbides (Cr 7 C 3, Cr 3 C 2 , Cr 23 C 6 ), iron-nickel (FeNi 3 ) and chromium-nickel (Cr 3 Ni 2 , CrNi), iron carbide (Fe 7 C 3 ) can be seen in XRD test. The decarburization of Cr 3 C 2 particles during microwave hybrid heating forms Cr 7 C 3 and Cr 23 C 6 . Cr3Ni2 and FeNi 3 phases might be due to the diffusion of chromium, nickel and iron elements from the substrate to clad at elevated temperature, which is a clear indication for metallurgical bonding of substrate to clad. The iron carbides phase (Fe 7 C 3 ) might be attributed to the dilution of iron elements from the substrate to the clad region during the microwave cladding. These phases were not noticed on the laser-clad surface, possibly due to the rapid solidification and diffusion rate being less than the microwave cladding process. As discussed in the EDS analysis, the clad powder Cr 3 C 2 -NiCr was observed to be completely intermixed and fused within the substrate. The developed clad surface must be properly mixed with the base material. Therefore diffusion rate of the substrate is unavoidable. The higher diffusion rate of microwave clad results in the gradual interaction between the substrate and clad powder, further forming the iron carbides. The formation of these iron carbides indicates the cause of excellent metallurgical bonding during hybrid microwave heating. Finally, it is observed from both processes that there is a good amount of chromium carbides segregated on the developed clad layer along with intermetallic, which further helps to increase the hardness and wear resistance of the developed coatings of both processes.
Microstructural observation. The microstructural study supports understanding the different phases present, their composition, grain boundary, inclusion, porosity, etc., appearing in the substance under examination. It helps to examine the microstructure's influence on clads' different properties. As a result, studies have been conducted on the microstructures of the developed clad. The microstructures of the Cr 3 C 2 -NiCr-based composite laser clad are shown in Fig. 7. The structure is completely dense interdendritic with a nickel-rich alloy phase and dendrites with chromium carbide spaces. The developed microstructure is typically solidified, with carbides as dendrites and a rich Ni phase dominating interdendritic spaces. It is also noticed that various columnar dendrites are growing perpendicular to the interface layer and interdendritic structure in the bottom part of the carbide layer, and few dendrites are noticed in the intermediate part of the clad layer. It is reported that the characteristics of this type of typical structure are directly related to the solidification rate (R) and temperature gradient (G) of the liquid alloy in the laser melt www.nature.com/scientificreports/ pool. At the beginning of solidification, there was a larger G value and a small R-value in the bottom part of the clad layer. This value of G/R gradually reduced to zero closer to the surface with the solidification process, which further leads to the cause for the above crystal growth 29 . Some coarse columnar dendrites were replaced by tiny dendrites covered by a bright eutectic. This was caused by the extremely high melting point of Cr 3 C 2 particles, which were abundantly present in the melted pool and would alter the temperature fields before the liquid-solid boundaries, affecting the solidification structure (Fig. 7c). The absence of microcracks and porosity has shown that the technical parameters considered for this study have ensured a high quality of the laser cladding process. A similar thick planar crystal zone between bonding and HAZ was also reported elsewhere 30 .
A typical cross-section of the developed microwave-clad cross-section is shown in Fig. 8. The microwave cladding process offers precise control over the heating parameters, such as power level and heating time, allowing for optimization of the cladding process. This control enables the formation of a desirable microstructure and facilitates the elimination of porosity, resulting in a pore-free structure with enhanced bonding strength. Compared to other cladding techniques, such as laser or thermal spraying, microwave cladding can achieve better metallurgical bonding. The unique characteristics of volumetric heating, rapid heating and cooling rates, and precise control over heating parameters contribute to the superior performance of the microwave clad, making  www.nature.com/scientificreports/ it stand out among alternative cladding methods. It is observed that the developed microwave-clad shows good bonding with the substrate by partial mutual diffusion of elements. A substrate-clad interface is free from any noticeable discontinuities. The observed wavy interface between the substrate and clad structure can be seen (Fig. 8a) due to localized melt pool current during microwave heating. The melting rate of clad powder and substrate directly depends on the melt pool current. It was also noticed that the developed microwave clads are free from observable pores, and interfacial cracks and clad regions appear defect-free. The magnified view of the clad section is shown in Fig. 8b, which shows that hard chromium carbide particles remain consistently dispersed within the soft nickel matrix. The nickel particles of the clad powder start melting first as microwaves initiate interaction during microwave heating, and hard carbide particles continue to be evenly distributed within the soft matrix Fig. 8b. The defect-free clad structure can be noticed due to the melt pool's slower solidifying rate. Various carbides, initially chromium carbide and other complex metallic carbides, are partly agglomerated due to the melt pool current and remain consistently disseminated. These carbides could further strengthen the developed clad structure and act as strengthening in the developed composite. The nature of the volumetric heating character is directly associated with hybrid microwave heating, which is affected by a minimal thermal grade in the exposed surface of the microwave. The carbides are distributed uniformly in the clad structure, which may result from the melt pool's slow solidification rate. Similar types of metallic carbides uniformly distributed are reported elsewhere 17,31 . The formation of cellular dendrites was not noticed anywhere in the developed microwave-clad structure. This could be due to a uniform thermal gradient that does not allow the cell to transition into dendrites 32 .

EDS analysis. An EDS analysis was conducted at various locations, and equivalent results are reported in
Figs. 9 and 10 correspondingly. As observed from the microstructure of the laser clad shown in Fig. 9a, three phases in the microstructure can be seen -grey, light white and light grey, mentioned as 1, 2 and 3, correspond-  (Fig. 9b) reveal that the occurrence of Cr and C influences the grey phase with impacts of approximately 76.86% and 9.87%, respectively. The existence of such hard metallic carbides (Cr x C Y ) in the laser clads, as discussed in section "XRD phase analysis", designates the prospects of showing better wear resistance. Meanwhile, the EDS study of point 2 (white phase) on the interface region denotes the existence of elements such as Cr, C and Fe (Fig. 9c). Point number 3 of the EDS study (light grey phase) of the interface, the region indicates the existence of the major elements such as Fe, Cr, C and Ni (Fig. 9d). A molten clad powder's clad layer can cause dilution of Fe and Mo, thus resulting in their presence. Thus, the clad comprises a relatively strong matrix (Fe-Cr-Ni based). Therefore, the uniform distribution of the carbides acts as reinforcements, which further helps to increase the wear resistance of tough metallic matrixes of the Cr 3 C 2 -NiCr-based composite laser clad. It can be observed that the fine dendritic structure of the interface region helps to improve the tribological properties. Earlier research has demonstrated that this kind of structure enhances the tribological properties of the target component. The major reason for this incidence is expected to be the dilution of Fe elements of clad powder and metallurgical bonding. Fe content was found in remelted coatings based on EDS analysis 33 .
As evidenced by the developed clad microstructure (Fig. 10a), two unique phases are observed-grain and the grain boundary, which showed as 1 and 2, respectively. Point number 1 signifies the grain and the developed clad matrix influenced by the occurrence of Fe, Ni and Cr with distributions of roughly 56%, 10%, and 23%, respectively, as shown in Fig. 10b. It is observed that a higher percentage of iron is attributed to the melting pool of molten clad powder caused by localized convective currents, which further leads to elemental interaction between clad powder and the target substrate. Which clears, the developed clad comprises a tough matrix (Fe-Ni-Cr). Further, Fe, Ni, and Cr had formed the intermetallics such as Cr 3 Ni 2 , FeNi 3 is observed in the XRD studies of microwave clad (Fig. 6b). The EDS studies of point 2 (Fig. 10c) marked on grain boundary enriched with Cr and C contribute roughly 83% and 10%, correspondingly. This indicates that the grain boundary of the developed clad has metallic carbides reinforced in the developed clad matrix. The occurrence of metallic carbides in developed microwave clad signifies the probability of performing superior hardness and resistance to erosion. Therefore, it is a clear sign from the EDS analysis that the uniform distribution of hard metallic carbides (Cr 3 C 2 , Cr 7 C 3 , Cr 23 C 6 ) acts as reinforcement in the tough matrix of microwave-clad (Cr 3 C 2 -NiCr), which are expected to provide resistance to wear at elevated temperature.

Microhardness of clads.
The microhardness was measured across the cross-sections of laser and microwave clad. It was pursued to know the microhardness variation across the developed clad layer and the base substrate. The microhardness distributions are illustrated in Fig. 11. The average microhardness of the substrate www.nature.com/scientificreports/ (T91) was 418 ± 12HV. However, the authors observed that the microwave-clad had hard chromium carbide particles consistently dispersed within the soft nickel matrix. The X-ray phase analysis of both the processes evidenced the common presence of phases like chromium carbides (Cr 7 C 3 , Cr 3 C 2 , Cr 23 C 6 ), Iron Nickel (FeNi 3 ) and chromium-nickel (Cr 3 Ni 2 , CrNi), despite these phases iron carbides (Fe 7 C 3 ) are observed in the developed microwave clads. The homogeneous distributions of such carbides in the developed clad structure of both processes indicated higher hardness. The typical microhardness of the laser-clad (1142 ± 65HV) was about 22% higher than the microwave-clad (940 ± 42 HV). As discussed in the EDS analysis, the clad powder Cr 3 C 2 -NiCr was completely intermixed and fused within the substrate (section "Microstructural observation".). Diffusion from the substrate is unavoidable because the developed clad surface must be mixed properly with the base material. Therefore, minor variations can be noticed in both clads' hardness profiles due to the Fe element from the target substrate. However, various waviness zones were seen in microhardness values across the sections; these non-uniform distributions across the segment leads to attributed to the alteration in hardness of the tough metallic matrix and hard carbide-based reinforcement, as well heating effects of both the process resulting in microstructural changes caused by successive clads developed through laser and microwave energy. It was reported experimentally that whenever hard carbide particles were fused into a softer surface, the hardness of the same surface was improved 34,35 . Many other scientists have also experienced similar behavior in the fusion of hard particles into softer surfaces, and the results were consistent with the findings of Li Pengting et al. 36 . It is also observed that the microhardness of laser clad is more, possibly due to the formation of a dendrite structure that limited the plastic distortion formed by the indenter. Hence, the developed laser-clad surface was thus strengthened by the dendrite structure. The residual stresses can also affect the microhardness of the developed clad. The laser cladding process often generates higher transient temperatures and thermal gradients, which can induce higher residual stresses. These compressive residual stresses can increase the microhardness of the laser clads 37,38 . The laser cladding process typically implies a faster cooling rate than the microwave cladding process. Laser cladding often involves faster cooling and rapid solidification, leading to fine and evenly distributed microstructures, such as fine dendritic structure, which is generally associated with higher microhardness 39,40 .
In the Microwave cladding process, with its unique heating process, a relatively slower cooling rate with different solidification behavior may result in different residual stress profiles, potentially leading to lower microhardness than the developed laser clads. However, The Cr 3 C 2 -NiCr based clads developed through both the process had a much higher microhardness than the target substrate, which may be primarily related to the formation of carbides (Cr 7 C 3 , Cr 3 C 2 , Cr 23 C 6, Fe 7 C 3 ).
Wear behavior of laser and microwave clads. The wear characteristics of the microwave-clad and laser-clad samples were evaluated through a linear reciprocatory ball-on-plate wear test. The test parameters, including load variations and sliding distance, are presented in Table 3. The investigation focused on the fretting/ fatigue wear behavior of the samples. During the initial test conditions, it was observed that the microwave-clad Figure 11. Shows Vickers's microhardness profile of Laser clad (black) and Microwave clad (red). www.nature.com/scientificreports/ samples exhibited a coefficient of friction (COF) of 0.80 µ compared to the COF of laser-clad samples of 0.61 µ. This can be attributed to the lower hardness of the developed microwave clads. However, as the test parameters increased, the COF of the microwave-clad samples drastically increased due to the tearing of the developed surface layer, as illustrated in Fig. 12c,d. On the other hand, the laser-clad samples demonstrated a lower COF as the load and sliding distance increased, as depicted in Fig. 12a,b. This suggests that chromium carbides in the laser clads acted as an internal lubricant, reducing friction coefficient and improving wear resistance. The comparison between microwave and laser clads regarding wear behavior revealed the different characteristics and performance of the two techniques. While microwave clads initially exhibited a lower COF, tearing the surface layer resulted in increased friction. In contrast, laser clad demonstrated a consistently lower COF, indicating its superior lubricating and wear-resistant properties attributed to chromium carbides. Figure 12a,b presents the worn-out images of the laser-clad samples. In contrast to the microwave-clad samples, the laser clad exhibited superior wear resistance under the test conditions. Hard carbide elements such as Cr 7 C 3 and Cr 23 C 6 played a significant role in preventing the detachment of melted particles from the surface, enhancing the resistance to wear. The uniform and robust surface of the laser clad resulted in minimal material loss during testing. The incorporation of carbides in the dendritic region increased hardness and wear resistance, albeit reducing the mean free path 40,41 . Figure 12c,d showcases the worn surface of the microwave-clad samples, which exhibited more surface damage and material loss than the laser-clad samples. Microcutting of the relatively soft binder, followed by carbide loosening and pullout, contributed to the material removal. Furthermore, at higher loads, fatigue-induced carbide fracture led to material loss. The main wear mechanisms observed during fretting wear were matrix flaking, carbide fracture, and pullout, possibly due to hard chromium carbide particles consistently dispersed within the soft nickel matrix.
The comparison between laser and microwave clads, as depicted in the worn-out images in Fig. 12, highlights the differences in wear behavior. Laser clads demonstrated greater wear resistance due to the effective retention of melted particles and a uniform and durable surface. On the contrary, microwave clads demonstrated greater surface damage and material loss, predominantly attributed to micro-cutting, carbide loosening, fatigue-induced carbide fracture, and the formation of a smooth layer. Figure 12d illustrates the development of a smooth layer during fretting wear, which entails the creation of a polished or relatively flat surface on the microwave clad under fretting conditions. This smooth layer is typically observed in the region of the worn surface and can be attributed to multiple factors. During fretting wear, the cyclic loading and relative motion between two contacting surfaces result in repetitive micro-slip and sliding at the interface. This motion leads to the removal of surface irregularities on the clad and the formation of wear debris. As the fretting process persists, the initial roughness www.nature.com/scientificreports/ and irregularities on the microwave-clad surface gradually diminish, resulting in a smoother surface appearance. Various factors, including material properties, contact conditions, and lubrication, can influence the formation of a smooth layer. In certain instances, protective films or oxide layers on the material surface can contribute to developing a smooth layer by acting as a barrier against further surface damage or wear 42 . These findings provide valuable insights into the wear mechanisms associated with laser and microwave cladding, emphasizing the advantages of laser cladding in terms of wear resistance and material preservation.

Conclusion
The current work institutes the probability of using microwave energy equal to the laser energy to develop Cr 3 C 2 -NiCr-based composite clads on the T91 steel substrate. Major observations are drawn from the current work as follows.
• Cr 3 C 2 -NiCr-based composite clads have been developed on the T91 steel substrate using laser and microwave irradiation. Both processes developed clads that showed excellent metallurgical bonding with the target substrate.
• Cr 3 C 2 -NiCr-based composite laser clads dense interdendritic structures with nickel-rich phases and dendrites with chromium carbide spaces. In the case of microwave clads, the hard chromium carbide particles consistently dispersed within the soft nickel matrix. • The clad spectrum of both the process evidenced the common presence of phases like chromium carbides (Cr 7 C 3, Cr 3 C 2 , Cr 23 C 6 ), Iron Nickel (FeNi 3 ) and chromium-nickel (Cr 3 Ni 2 , CrNi); despite these phases, iron carbides (Fe 7 C 3 ) are observed in the clads developed through microwave energy. • The Cr 3 C 2 -NiCr-based composite clads developed through both processes had a much higher microhardness than the target substrate, which may be primarily related to the formation of carbides (Cr 7 C 3 , Cr 3 C 2 , Cr 23 C 6, Fe 7 C 3 ). • The average microhardness of the developed clads of laser and microwave energy increased by 2.7 times and 2.3 times compared to the substrate's average microhardness (418 ± 18HV). • The microwave cladding process is a cost-effective, eco-friendly, and energy-efficient material processing method. • The study analyzed the wear behavior of microwave and laser-cladding samples using a linear reciprocatory ball-on-plate test. Microwave-clad samples had a slightly lower coefficient of friction but increased as load and sliding distance increased. Laser cladding showed a lower coefficient, possibly due to chromium carbide's internal lubrication. • The laser-cladding samples showed superior wear resistance due to hard carbide elements, preventing melted particles from detaching and resulting in a uniform, durable surface. Microwave-clad samples showed more surface damage and material loss, driven by micro-cutting, carbide loosening, and fatigue-induced carbide fracture. www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.